A known shock-absorption steering device (hereinafter referred to as a steering device) disclosed in JP3242877B includes a safety steering column having a column tube on which plural notches are formed on an peripheral surface at an end portion of the column tube in an axial direction thereof. The end portion fits to a conical component member that supports a bearing of a steering shaft.
In this configuration, in the event of a vehicle collision, the notches formed on the column tube starts splitting, and end portions of the column tube are plastically deformed so as to be curled up or split so as to be in a tulip shape in order to absorb the impact energy.
Another type of known shock-absorption device (safety steering column) is disclosed in JP3581136B. Such steering device includes two casing tubes, a first casing tube 3a and a second casing tube. The first casing tube has a larger diameter, and the second casing tube has a smaller diameter. In this configuration, the second casing tube partially fits into the first casing tube by means of a sliding piece.
The sliding piece is pressed to a surface of the second casing tube by an axial tension of a screw that is screwed to the first casing tube.
In this configuration, by adjusting the axial tension of the screw, a displacement force between the first casing tube and the second casing tube, in other words, a load by which the column is retracted in the event of a vehicle collision, is adjusted.
According to the shock-absorption steering device disclosed in JP3242877B, the load in the event of a vehicle collision can be adjusted by changing the number of the notches formed on the column tube or the thickness of the wall of the column tube, however, if the number of the notches is increased or the thickness of the wall is reduced in order to set the level of the load to be low, rigidity of the column tube may be reduced, and discomfort vibrations are more likely to occur at the steering device.
Further, the load is varied depending on the collision stroke by varying the thickness of the wall of the column tube in an axial direction thereof, however, such column tube is not easy to produce.
According to the shock-absorption steering device disclosed in JP3581136B, the load is varied depending on the impact stroke by varying an external diameter of the second casing tube having the smaller diameter, or by varying a friction resistance. Specifically, the friction resistance is varied by varying a roughness of the sliding surface on which the sliding piece slides.
However, even when the load is changed by the above method, it is difficult that the difference of the sliding load is minimized, and such steering tube is not easy to produce.
A need thus exists to provide a shock-absorption steering device by which a level of the load to be absorbed in the event of a vehicle collision can be easily set with high flexibility, the shock-absorption steering device being easily produced.